The present invention pertains to the automatic inspection of manufactured articles or objects.
The constantly decreasing size and increasing complexity of electronic circuits challenge the state of the art in a number of technical fields. One of these fields involves the inspection of manufactured circuits for defects such as open-circuited conductors or adjacent conductors shorted to each other. An exemplary manufactured circuit may comprise a ceramic substrate about 2.5cm wide by 12.5cm long, containing plated silver or copper conductors spaced about 0.3mm apart. The conductors may have corners and junctions. They may terminate at pads for mounting individual semiconductor chips or for external package pins; they may also terminate at via holes which join conductors on the other side, for instance. When circuit substrates of this type are designed, the conductors, pads, etc. are laid out in a grid of small cells or areas which may be, e.g., squares 0.15mm on a side.
Inspection of such an object requires a high-resolution image, to detect hairline conductor cracks, dendritic bridges between conductors, and other small defects. But the direct comparison with a master or ideal pattern at this high resolution has two disadvantages. First, a high-resolution master pattern would require almost a million bytes of storage for the above exemplary substrate, even if each pattern cell required only a single bit. Second, it is highly unlikely that a direct comparison would produce an exact match, even if the object contains no defects at all. Rough edges on conductors, stray isolated bits of foreign material, noise from the scanning process, etc. would all produce a significant number of apparent errors. Therefore, it is not feasible to inspect such an object by direct comparison with a master pattern.